The optical disk is receiving a lot of attention as a recording device capable of recording large quantities of information. However, the recording/reproducing apparatus of the optical disk presents the disadvantage that its access speed is slow compared with a conventional magnetical disk apparatus, and the improvement of this access speed has been a subject of research in recent years.
The structure of an optical disk will be described with reference to FIG. 7.
Tracks 22 are formed in a concentric or spiral manner on the disk surface of an optical disk 21. The FIG. 7(a) illustrates the case of concentric tracks 22. These tracks 22 are, as illustrated in detail in FIG. 7(b), long continuous physical protuberant portions which cross sections are trapezoidal (hereinafter referred to simply as protuberant portions), previously formed on the disk surface. Or these tracks 22 may be, as illustrated in detail in FIG. 7(c), long continuous phYsical variations previously formed on the disk surface by modifying the composition of the materials in specified sections of the recording surface, by making these sections only in a crystallized phase and the other sections in an amorphous phase, or the like. The write once type or rewritable type optical disk 21 is arranged so that the information from a user is recorded on the tracks 22 or in the intervals between the tracks 22. Moreover, ID sections 23, wherein information such as the track number, a synchronizing signal is previously recorded, are in some instances formed at a proper location on each of these tracks 22, by interrupting intermittently the protuberant portions of the disk surface as shown in FIG. 7(b), or by modifying intermittently physical property such as the variation of the reflectance ratio or the phase transition, as shown in FIG. 7 (c).
When the optical disk 21 is loaded in the recording/reproducing apparatus, a light beam 24 from an optical head, not shown, is irradiated on the disk surface, as shown in FIG. 7(b). The light beam 24, beside executing the recording and the reproduction of the user's information, is also responsible for reading the information recorded in the above-mentioned ID sections 23 and for obtaining a tracking error information, through the variation in the amount of light of a reflected light from a track 22. When the random access of the user's information is carried out, the optical head is moved and slides in the radial direction of the optical disk 21, and the light beam 24 is controlled so that it is irradiated on a prescribed track 22, or on a prescribed interval between two tracks 22.
Hence, in order to access the optical disk rapidly, as mentioned earlier, a track counter for counting the number of tracks 22 the light beam 24 passes upon, and for detecting that the light beam 24 reached the prescribed track 22 when the optical head moves and slides, becomes indispensable.
The operation of a conventional track counter will be described with reference to FIG. 8.
In the optical head, a RES signal indicating the tracking error and a REF signal indicating the increase or reduction (variation) in the amount of reflected light, may be obtained from the reflected light of the light beam 24. The RES signal is a signal which detected that the light beam 24 deviated from a track 22 or from the center of an interval between two tracks 22 through the widely known 3-beam method or push-pull method, and is used in the tracking servo.
The REF signal is a signal which detected the variation in the amount of light of the reflected light from an optical disk 21, and is used for reading the information recorded in the above-mentioned ID sections 23 and the like.
As shown in FIG. 8, when the light beam 24 passes consecutively upon the tracks 22 as the optical head moves and slides, the RES signal shows a substantially sinusoidal waveform which equilibrium state is the zero level. As for the REF signal, it shows a substantially sinusoidal waveform which equilibrium state is the reference voltage Vr. The RES signal and the REF signal have a phase difference of almost .+-.90.degree. according to the passage direction of the light beam 24. Moreover, as in reality the optical disk 21 is rotating, the light beam 24 passes upon the tracks 22 not at right angles, but in the diagonal direction with respect to the longitudinal direction of the tracks 22.
Among the points where the level of the RES signal equals zero (hereinafter referred to as zero cross points), the zero cross points where the RES signal passes from a positive to a negative direction, indicate that the light beam 24 is positioned on the center of an interval between two tracks 22, as it is clearly illustrated the figure. Hence, if a ZC signal in which a pulse rises at each of these zero cross points, is generated, it becomes possible to detect the number of tracks 22 the light beam 24 passes upon by counting the pulses of the ZC signal.
Also, by checking whether the REF signal is higher or lower than the reference voltage Vr at a zero cross point where the above-mentioned RES signal passes from a negative to a positive direction, there may be detected whether the phase difference of both signals is positive or negative. For example, by generating a DIR signal which is in the high level when the REF signal is higher than the reference voltage Vr and in the low level when the REF signal is lower than the reference voltage Vr, the passage direction of the light beam 24 may be detected based on the DIR signal.
Thereby, up to now, for example the count up or down of an updown counter used to be determined according to the DIR signal, and the number of tracks 22 the light beam 24 passed upon used to be detected by counting the pulses of the ZC signal with this updown counter.
However, in some instances, ID sections 23 are formed on specific positions as mentioned earlier, on each track 22 of the optical disk 21 When the light beam 24 moves along a track 22, at these ID sections 23, as shown in FIG. 9, the interference action of the reflected lights varies because of the interruption in the protuberant portion and the REF signal has pulsations centered on the reference voltage Vr. Thereby, the track number information and the like, recorded in the ID sections 23 as outlined earlier, may be read by detecting whether this REF signal is higher or lower than the reference voltage Vr.
However, when such ID sections 23 are formed on each track 22, as shown in Fig. 10, when the optical head moves and slides and the light beam 24 crosses the ID sections 23, turbulence is generated in the RES signal and the REF signal. That is, the zero cross points of the RES signal become ambiguous, since the tracking error cannot be detected in the interrupted sections of the protuberant portions of the tracks 22, as shown in the figure. The waveform of the REF signal is also disturbed on a large scale for the similar reason. However, as the turbulence of the waveform of this REF signal is further complicated, it is omitted in the figure.
Therefore, an accurate number of tracks 22 the light beam 24 passed upon cannot be detected when the light beam 24 crosses the ID sections 23, by merely determining the count up or down based on the DIR signal and by counting the pulses of the ZC signal, like in a conventional track counter. Moreover, such a thing does not happen only when ID sections 23 are formed on the tracks 22, but also when a turbulence is generated in the waveforms of the RES signal and the REF signal because of a scratch on the optical disk 21 or the like.
Accordingly, the conventional track counter for optical disk used to present the problem that the improvement of the information access speed was hindered, because the light beam, controlled based on the number of tracks it passed upon, could not be moved quickly to a desired track.